Although various systems such as a gun drilling system, an ejector system (double tube system), a single tube system, etc., have been publicly known as a deep-hole drilling system, the gun drilling system is suitable for deep-hole drilling of a small diameter. That is, the gun drill system generally employs a gun drill having a cutter head, the outer cross section of which is a notched circle (⅔ through ¾ circle), provided at the distal end of a hollow shank portion having the same notched circle, carries out cutting while supplying coolant, which is supplied through the interior of the hollow shank portion, from a coolant discharge port at the distal end face of the cutter head to a cutting portion, and discharges cutting chips, which are generated in line with the cutting, to the exterior through a cutting chip discharge groove whose section is V-shaped, along the lengthwise direction of the outer circumference of the hollow shank portion together with the coolant, wherein since the coolant supplying pressure is determined by the tool length even if the cutting hole becomes deep, the gun drill system is suitable for deep-hole drilling of a small diameter.
Conventionally, as the above-described gun drill, one for which a cutter head consisting of a solid cemented carbide alloy is brazed to the distal end of a steel-made hollow shank portion is generally used. In addition, it is general that, in particular, in a small-diameter gun drill, if the blade is worn to some degree, the blade is re-used after polishing the same.
However, in recent years, it is keenly desired that the efficiency of cutting including deep-hole drilling is increased, and at the same time, labor and cost required for improvement of durability and maintenance of cutting tools and other components are reduced. In this connection, the cutting efficiency and durability are insufficient with the conventional gun drill. In particular, labor and time required for polishing the cutter having a small diameter become a problem.
The present invention was developed in view of the above-described situations, and it is therefore an object of the invention to provide a gun drill having excellent cutting efficiency, by which the cutting rate can be further significantly improved in comparison with conventional gun drills, and having a prolonged blade service life, capable of reducing the frequency of polishing for re-use in a case of a small diameter, and capable of accordingly reducing labor and time.